1. Field of the Invention
This invention relates to a dynamic pressure bearing apparatus, and more particularly, to a bearing apparatus suitable for bearings to be used in a rotatry unit such as a rotary polygonal mirror type light deflector in a laser beam printer, a rotary head in a video tape recorder and the like.
2. Related Background Art
Conventionally, there has been presented a dynamic pressure bearing apparatus as shown in FIG. 1A. In this apparatus, a shaft 1 which rotates while supporting a rotary unit is rotatably mounted into a sleeve 2 in fluid such as lubricating oil. On the cylindrical surface of the shaft 1 (hereinafter referred to as a radial surface), first shallow grooves of a herringbone shape 4 and 5 are formed. When the shaft 1 is rotated, a dynamic pressure in a radial direction (a direction directed to an outer periphery from a central axis) occurs. As a result, a non-contact state between the shaft 1 and the sleeve 2 is maintained with respect to the radial direction. A distribution profile of the dynamic pressure in the radial direction is illustrated in FIG. 1B. A bottom surface of the sleeve 2 opposed to the end surface 6 of the shaft 1 is defined by an insert member 8 fitted into a bottom recess of the sleeve 2. On the surface of the insert member 8, a second shallow groove 9 of a spiral shape is formed so that a dynamic pressure in a thrust direction occurs when the shaft 1 is rotated. Thus, also with respect to the thrust direction, a non-contact state is maintained between the shaft 1 and the sleeve 2. A distribution profile of the dynamic pressure in the thrust direction is illustrated in FIG. 1C.
An inner groove 10 is formed in an inner side wall of the sleeve 2 near an opening end. A third shallow groove 11 of a spiral shape is formed on a portion of the radial surface of the shaft 1 located nearer to the opening end than the inner groove 10. As a result, when the shaft 1 is rotated, the third shallow groove 11 acts to feed the fluid 3 from the opening end of the sleeve 2 to the inner groove 10 with pressure so that the fluid 3 is prevented from spilling from the opening end of the sleeve 2.
In the prior art apparatus, however, when the shaft 1 is rotated, a pressure in the thrust direction is also produced by the second shallow groove 9 spirally formed on the insert member 8. Therefore, in order to maintain the non-contact state of the shaft 1 by floating the shaft 1 above the insert member 8, the degree of square, i.e., the flatness, of the surface of the insert member 8 on which the shallow groove 9 is formed, with respect to the center axis of an inner diameter of the sleeve 2, need be high. At the same time, the degree of square of the end surface 6 of the shaft 1 which is opposed to the surface of the insert member 8, with respect to the center axis of the shaft 1, should be high.
If those degrees of square are insufficient, a floating force becomes weak, and simultaneously a peripheral edge of the shaft end 6 strikes the surface of the insert member 8. Thus, the speed of rotation is disturbed and the axis tilts. This produces negative influences on the rotary polygonal mirror type light deflector, as opposed to the desired high rotational accuracy.
In general, it is required that the degree of square of the surface of the insert member 8 on which the shallow groove 9 is formed be 2-3 .mu.m, and that of the end surface 6 of the shaft 1 also be 2-3 .mu.m. This linear measurement is the difference between the highest and lowest points on the end surface. Therefore, a high precision operation is needed for fabricating the insert member 8 and the sleeve 2, thus making the costs of these components quite high. Further, there is also a problem that it is difficult to make the insert member 8 with plastics, which is very effective to reduce the cost of a dynamic pressure bearing apparatus.